The invention relates to a method for grinding the main and rod bearings of a crankshaft by external cylindrical grinding in a grinding machine and to a grinding machine for carrying out the method. A method and a grinding machine of the type mentioned are known from DE 10 2008 007 175 A1.
It has already been proposed in EP 1 181 132 B1 to finish-grind the rod bearings before the main bearings during the external cylindrical grinding of the main and rod bearings of a crankshaft. This proposal is based on the knowledge that the considerable deformations of the crankshaft during the grinding of the rod bearings can be at least partly eliminated again during the subsequent finish-grinding of the main bearings. However, it was assumed here that the rough-grinding of the main bearings still had to take place before the grinding of the rod bearings. Therefore, according to EP 1 181 132 B1, first of all a steady-rest seat has to be initially ground onto a main bearing of the crankshaft, in order that the main bearings can be rough-ground with the required accuracy. To this end, the crankshaft has to clamped with a precisely defined rotational axis, specifically its defining geometrical longitudinal axis, which is the defining reference axis for all the main bearings with regard to diameter, roundness, true running and centricity. This defining geometrical longitudinal axis also has to be available as reference axis for the machining of the rod bearings. Following the rough- and finish-grinding of the rod bearings, the main bearings of the crankshaft are finally finish-ground. The method known from EP 1 181 132 B1 has the advantages that all of the grinding operations can be carried out in a single setup.
However, the constraints that arise on account of the clamping and supporting of the crankshaft during grinding have introduced the risk of other deformations, as is described in detail in DE 10 2008 007 175 A1. Therefore, as a remedy, that citation proposed giving up the grinding of the crankshaft in a single setup. Rather, according to DE 10 2008 007 175 A1, two grinding stations, which can be located within a single grinding machine, are proposed. First of all, the rod bearings are rough- and finish-ground in the first grinding station. Subsequently, the crankshaft is transferred into the second grinding station, in which the main bearings are rough- and finish-ground. The particular feature of the known method is that the crankshaft to be ground is clamped in the two grinding stations with its rough contour merely machined by chip removal. In this case, the cylindrical circumferential surfaces of the crankshaft are machined primarily by turning, drilling or trochoidal milling, that is to say in a still unground state. In the first grinding station, the crankshaft is mounted in this case in shell chucks which are attached advantageously to end-side cylindrical portions or to the two outer main bearings of the crankshaft. Naturally, during the grinding of the rod bearings, the crankshaft does not rotate about its defining geometrical longitudinal axis but about a rotational axis that deviates therefrom and is given by the rough contour of the crankshaft at the clamping points. Since, however, the rod bearings have to be ground anyhow by CNC-controlled external cylindrical grinding in the pin-chasing grinding process, according to DE 10 2008 007 175 A1, a corresponding correction in the computer of the grinding machine has to be made. To this end, the crankshaft has to be measured precisely before grinding. When the deviations of the actual rotational axis from the defining geometrical longitudinal axis of the crankshaft are known, this can be sensed by computer and taken into account during CNC grinding. As a result, a crankshaft which has as yet unground main bearings but the rod bearings of which have been ground as if the crankshaft had been rotated about the exact defining geometrical longitudinal axis is present following grinding in the first grinding station.
According to DE 10 2008 007 175 A1, it is only in the second grinding station that the crankshaft is clamped between centers which penetrate into the usual centering bores in the end faces of the crankshaft. These centering bores are made by the crankshaft manufacturer even before the rod bearings are ground and determine the defining geometrical longitudinal axis of each crankshaft.